Apparatus for continuously digesting alumina

ABSTRACT

ALUMINA IS DIGESTED BY THE BAYER PROCESS INTHE TUBE SIDE OF A TUBE-AND-SHELL HEAT EXCHANGER HEATED BY STEAM FLASHED FROM THE DIGESTED SLURRY. A PLURALITY OF SEPARATE SHELLS ARE ARRANGED END-TO-END WITH TUBE SHEETS BETWEEN THEM AND WITH THE TUBES EXTENDING THROUGH THEM IN SERIES. EACH TUBE IS SEPARATELY VALVED FOR SIMULTANEOUS WASHING OF SELECTED TUBES WHILE THE OTHER TUBES ARE ON STREAM.

June 22, 1971 A. JUHASZ 7 ET AL APPARATUS FGR CONTINUOUSLY DIGESTING ALUM'INA 2 Sheets-Sheet 1 Filed April 21, 1969 June 22, 1971 A. JUHASZ ETAL 3,586,487

APPARATUS FOR CONTINUOUSLY DIGESTING ALUMINA Filed April 21, 1969 2 Sheets-Sheet 2 Fig. 3

United States Patent 01 3,586,487, Patented June 22,, 1971 1m. or. nine 11/00 U.S. Cl. 23--267 4 Claims ABSTRACT OF THE DISCLOSURE Alumina is digested by the Bayer process in the tube side of a tube-and-shell heat exchanger heated by steam flashed from the digested slurry. A plurality of separate shells are arranged end-to-end with tube sheets between them and with the tubes extending through them in series. Each tube is separately valved for simultaneous washing of selected tubes while the other tubes are on stream.

With the generally and worldwide applied Bayer process, alumina i.e. aluminium oxide is dissolved from bauxite by sodium aluminate liquor, and this stage is known as the digestion of bauxite. The efiiciency (yield and exploitation) depends first and foremost on the digestion temperature; on the other hand a higher temperature requires a higher pressure. There are some bauxite types (e.g. diaspore containing bauxites) which can be digested with an acceptable efficiency only at a high temperature above 250 C. However, in the traditionally used autoclaves such a high temperature cannot be realised, because the corresponding pressure (over 50 atm.) cannot be maintained economically on an industrial scale.

A number of researchers have made trials with high temperature digestion (250300 C.) and several of them have come to the conclusion that this may be realised in tube-formed autoclaves i.e. tube digestion.

Generally the practical solution was thought to be carried out in a double-walled tube to which slurry is charged at one end by means of a high pressure pump into the inner tube while at the other end of the tube a valve regulates the necessary pressure; meanwhile the heating medium is conducted into the outer tube which serves to heat the slurry. In practice this solution can be carired out, however it has several drawbacks, as follows (1) The heating surface per unit length is relatively small, therefore a rather long double-walled tube is necessary.

(2) The outer surface (in connection with the environment) of the heating surface per unit length is rather large, and therefore heat-loss is considerable.

(3) For cleaning purposes the equipment has to be completely put out of service periodically.

(4) Within a working period heat transfer and along with same the capacity gradually decreases on account of scale, resulting in an irregular production, and at the same time utilization of the connected work-units and equipment are detrimentally affected.

The present invention provides such a heating system which renders modern high-temperature digestion possible by eliminating the above discrepancies.

In the case of the equipment of the present invention:

1) The drawback resulting from the small heating surface of the inner tube of the double-wall piping has been eliminated by replacing the same with several small diameter pipes by which the heating surface per unit length has been increased by three to tenfold.

(2) 'On the basis of the foregoing item the outer surface pertaining to one heating surface unit considerably decreases thus resulting in a lower heat-loss.

(3) The equipment has not to be put out of service for the purpose of cleaning as it can also be cleaned during operation.

(4) By means of the cleaning method mentioned in the foregoing item both the heat transfer and the capacity of the equipment can be kept on a nearby constant value. This results in a uniform operation which in case of continuous working represents a great advantage.

The present invention is a digesting equipment for the continuous processing of alumina containing minerals, the heating elements of which form a series of single tube heat-exchangers; between two adjacent heat exchangers linked together there are some perforated connecting and packing plates ensuring the uniaxiality, according to the layout of the heating tubes, or else there are only perforated packing plates ensuring the uniaxiality, according to the layout of the heating tubes, or else there are only perforated packing plates. On the slurry side locking head of the first and last element of the heating element series there is a special locking device for every heating tube. The slurry fed into the heating tubes is suitably heated up to a temperature of over 250 C. by means of flashingor fresh-steam, condensate water, chemical solutions or other heating media conducted into the heating area of the heat exchangers. One may also conduct slurry into the heating area of some heating elements in which case the digested slurry does not get into the flash tanks (neither partly nor completely), but the slurry to be digested is heated up by means of slurry/slurry heat-exchange.

The invention will be described hereinafter in greater detail with reference to the accompanying drawings in which:

FIG. 1 is a schematic illustration of the beginning and the end of the heat exchanging series.

FIG. 2 shows a connection of two heat exchangers following each other.

FIG. 2a is a variation of the above connection.

FIG. 3 is a schematic illustration of the connection for the digesting equipment.

FIG. 1 shows the beginning and the end of the digesting heat exchanging series according to the invention. Within the part of the shell marked 1 of the heat exchanger there are for instance seven heating tubes marked 2. Within these is flowing the slurry being under digestion, while the area between the shell and the tubes is filled with the heating medium. On both ends of each of the heating tubes marked 2, there are 22 valves: On the slurry charge side slurry valves 7 -7 connected to the delivery duct of the slurry marked 3; and the wash liquid valves 9 -9 connected to the wash liquid inlet conduit 5; on the exit side are the slurry valves 8 -8 connected to the tube of the digested slurry marked 4, and finally the wash liquid valves 10 -10 connected to the wash liquid outlet conduit 6.

FIG. 2 shows the connection ensuring the uniform slurry flow rate in the two heat exchangers following each other. Flange 11 of shell 1 contains the tube plate 12 holding together heating tubes 2. Between the two flanges 11, respectively the tube plates 12 one may see a connecting plate 13 and on both sides of the same a packing plate 14. The hole distribution of the connecting plate 13 and the packing plates 14 is identical with that of the tube distribution of the tube plate 12.

FIG. 2a shows a variation of the former connection at which the connecting plate 13 and one of the packing plates 14 has been eliminated and only one packing plate 14 ensures a tight connection.

FIG. 3 shows a connecting diagram of the digesting equipment of the present invention. Characteristic elements of the equipment are: feeding pump 15, slurry charging duct 3, single heat exchanging series 16 connected to one another on the slurry side, digested slurry duct 4, throttle valve 17, flash tanks 18, expanded slurry duct 19, expanded steam duct 20, fresh steam duct 21, feeding and discharge conduits 5 and 6 for wash liquid, exit pipes of condensate 22 for cooling medium.

Operation of the equipment: Through pressure tube 3 pump continuously feeds the slurry to be digested into the heating tubes across slurry feeding valves 7 -7 in which the slurry is flowing at a steady rate from the feeding end of the line up to the exit, While the chemical process of the digestion is taking place. The digested slurry exits through valves 8 -8 into slurry duct 4 and then passes through throttle valve 17 into the flash system 18 1920. Heating of the heat exchangers is effected by expansion steam flowing through ducts 20 and by fresh steam flowing through ducts 21. Condensate is discharged through ducts 22. Cleaning of the cooling tubes of the heat exchanging line is effected by wash liquid introduced through ducts 5. During the continuous digesting operation slurry is conducted generally through six of the heating tubes 2, so that there might for example by slurry flow through valves 7 -7 7 -7 8 8 and 8 -8 Valves 7 and 8 are closed but that for the wash liquid 9 and 10 is open across which wash liquid flushes one of the heating tubes. Valves 9 -9 9 -9 10 -10 and 10 -10 for wash liquid are closed. In such a manner any of the seven heating valves can be switched out and Washed; while across the remaining six ducts digestion is taking place without any disturbance.

For our tests on an industrial scale the flow rate has been set at 1.5-1.8 m./mp., however measurements have been carried out at lower and higher flow rates too. Experiments have shown that the heat transfer varies sensitively with the flow rate.

In consequence of the quick heating up, in the case of some bauxite types, the dissolved silicic acid does not separate in a suitable measure from the digesting liquor, therefore after the temperature has been attained the slurry has to be held for a longer or shorter time in order to arrive at an adequate alumina quality. From the point of view of the completion of the digestion holding is also advantageous. For holding the time spent in the flash tanks is suflicient from time to time, but if need be a holding autoclave can be provided between the heating element series and the flash tanks.

As an example, there can be provided a digesting line suitable for the digestion of a slurry quantity of 100-125 'mfi/h. The heating elements 16 seen in FIG. 3 may each have a length of 6.5 m. and be constructed of a tube of 267 x 8 mm.. In one heating element 19 heating tubes of 38 x 2.5 mm. 5 are placed, having a total heating surface of 12 m? If twenty-five of the individual heating elements are connected in series the slurry temperature of -125 m. /h. can be increased from 100 C. to 275 C. with a suitable heating medium. The total tube length is thus 170 m. According to our experiments the heat transfer factor at the start is 2500 kca1./m. /h. C. By means of constant Washing at any heating tubes this value may be kept at an average of 1800 kcaL/mF/h. C.

What we claim is:

1. Apparatus for the continuous digesting of alumina, comprising a plurality of shells arranged in end-to-end relationship and separated from each other by tube sheets, a plurality of tubes passing through the shells in series and through the tube sheets, means to supply heating fluid to the interior of the shells in parallel, means for selectively individually supplying alumina slurry to the interior of the tubes, and means for selectively individually supplying wash liquid to the interior of the tubes.

2. Apparatus as claimed in claim claim 1, said supply means for slurry and wash liquid comprising separate slurry and wash liquid supply conduits, and valve means individual to each said tube for selectively directing slurry or Wash liquid through the associated tube.

3. Apparatus as claimed in claim 1, and means for separately introducing said heating fluid into each of said shells.

4. Apparatus as claimed in claim 3, and means for flashing off steam from the digested slurry in a plurality of stages, and means for introducing steam flashed off from said stages into said shells in a sequence such that the slurry to be digested is in heat exchange with the successive portions of steam in the reverse order in which the portions of steam are flashed from the slurry whereby the slurry moving through the tubes comes into heat exchange with successively hotter portions of flashed steam.

References Cited UNITED STATES PATENTS 2,886,483 5/1959 Rosenblad -95 CHARLES SUKALO, Primary Examiner U.,S. Cl. X.R.

233l2ME; 165-95, 101 

